In order to practically achieve control or protection of an electric power apparatus in a transformer substation, operational conditions of apparatuses need be grasped by measuring a current or voltage waveform in a body of the electric power apparatus. In a conventional system, conversion is performed into a current value or a voltage value suitable for measurement by a transformer (a current transformer or a voltage transformer) attached to the body of the electric power apparatus. Thereafter, connection is made from the body of an electric power apparatus to a protection control apparatus which is separate from the body of the electric power apparatus, by a lot of electric cables, and waveforms are then measured.
On the other side, application of a process bus has already been started as a method of obtaining current or voltage waveform data, in accordance with development in communication and digital control technologies in recent years. In a system employing the process bus, a merging unit is provided near the electric power apparatus, and waveform signals from a plurality of transformers are once inputted into this merging unit.
The merging unit merges a plurality of waveform signals as digital data, and transmits the data to a superordinate unit, such as a guard relay or a line control unit (Bay Control Unit), by serial communication called a process bus. The superordinate unit transmits a close command or a open command for a circuit-breaker as a message on the process bus to the circuit-breaker control unit provided near the circuit-breaker.
In this manner, reduction in volume of wiring of electric cables, lightening of load on a secondary side of a transformer, and standardization of a protection control system can be achieved. In a conventional system, for example, a secondary rated current of a current transformer is standardized at 1 A or 5 A. In a similar manner to above, communication on a process bus is normalized/standardized under the IEC61850 standard, and the process bus is applied in accordance with the standard. Thereby, digital data can be shared among units of different manufacturers, i.e., interactive operation can be achieved.
There has been conventionally known a method of restricting occurrence of a transient phenomenon, which is a server burden to an electric power system or electric power apparatus, by controlling timing for close or open of a circuit-breaker. As an apparatus using the method, there is a phase control switching apparatus which controls an open timing of a main contact of a circuit-breaker at the time of opening the circuit-breaker so as to come at a target phase of a breaking current, and controls a closing timing of the main contact of the circuit-breaker at the time of closing the circuit-breaker so as to come at a target phase of a power-supply voltage waveform.
In order to cause the circuit-breaker to close or open at a target phase when a close signal or an open signal for the circuit-breaker is received, this phase control switching apparatus has a function to delay a command output timing to the circuit-breaker, i.e., a conduction start timing to a close coil or a trip coil of the circuit-breaker. Such a switching control method for a circuit-breaker is often referred to as phase control switching.
Next, an example configuration of a conventional breaker-phase-control switching method which employs a phase control switching apparatus and a process bus does not employ a process bus will be described with reference to FIG. 6. A circuit-breaker close command 82 or an open command 83 from a superordinate unit 22, such as a line control unit or a guard relay, is fed in form of a voltage as a contact signal to the breaker-phase-control switching apparatus 81 via an electric cable 61A or 61B.
The breaker-phase-control switching apparatus 81 predicts an operating time since electric conduction to a closing coil 33 or a trip coil 34 of a circuit-breaker 31 until completion of close operation or open operation of the circuit-breaker, and switches on a transistor 2A or 2B, preceding a target close phase or open phase by the predicted operating time. The close coil 33 or trip coil 34 of the circuit-breaker is thereby electrically conducted via the electric cable 62A or 62B, and the circuit-breaker 31 operates (close or open).
Though varying depending on purposes and electric power apparatuses as targets, in order to effectively achieve phase control switching, control accuracy (difference between a target phase and an actual switching phase) needs to be fall within approximately ±30 degrees including accuracy on the side of a circuit-breaker. For this purpose, variants of timing at which the breaker-phase-control switching apparatus 81 starts electrically conducting the close coil 33 or the trip coil 34 need to be sufficiently smaller than a time equivalent to phase angles of ±30 degrees.
Specifically, variants need to be within several hundred ±μs. In case of this example configuration, a current waveform from a current transformer 42, a voltage waveform from a voltage transformer 41, and electric conduction 54 to the coil of the circuit-breaker 31 reach immediately as electric signals instead of communications. Therefore, phase-control switching can be performed with high control accuracy, without considering a transfer delay of a signal or variants thereof.
On the other side, when phase-control switching is performed in a transformer-substation-protection control system which employs a process bus, waveforms and commands are transferred by communications as sample values and messages, respectively. Therefore, control accuracy is influenced by these transfer delays and variants thereof.
FIG. 7 shows conception of phase control switching provided under the IEC61850 standard. CSWI 71 expresses a function of a protection relay. CPOW 72 expresses a function of a breaker-phase-control switching apparatus. XCBR 73 expresses a function of a circuit-breaker. TCTR 74 expresses a function of a current transformer. TVTR 75 expresses a function of a voltage transformer.
Phase control switching is performed as follows. Firstly, a close command or an open command for a circuit-breaker is transmitted from CSWI 71 to CPOW 72. CPOW 72 issues a time-limit operation command (Time-activated Operate) to the circuit-breaker with reference to a current-waveform sample value from TCTR 74 or a voltage-waveform sample value from TVTR 75. XCBR 73 starts an internal timer in response to this command, and performs close operation or open operation for the circuit-breaker when the time limit comes, thereby completing phase-control switching operation. Here, a transmission delay exists when transmitting a time-limit operation command from the phase control switching apparatus to the circuit-breaker. Therefore, control accuracy is influenced.
In order to specifically describe the problem described above, an example configuration of a conventional breaker-phase-control switching system in a transformer-substation-protection control system which a process bus is applied will be described with reference to FIG. 8. In the present example configuration, the superordinate unit 22 described above has functions of logic nodes CSWI and CPOW as described above. The circuit-breaker control unit 1 has only the function of XCBR which controls the circuit-breaker 31.
The superordinate unit 22 receives current or voltage waveform data 51 (Sampled Value) transmitted by the merging unit 21 and performs control and measurement operation. The superordinate unit 22 sets a value of a timer so as to start close or open operation of the circuit-breaker 31, preceding by a predicted switching operation time for the circuit-breaker 31, and issues the close or open command 52 as a time-limit operation command for the circuit-breaker, to perform phase control switching.
The time-limit operation command is transmitted from the superordinate unit 22 to the circuit-breaker control unit 1 via the process bus 24, close or open operation is detained by delay relative to a set timer value, owing to existence of transfer delay of the close or open command.
Other data communication of current or voltage waveform data 51 than the time-limit operation command also exists on the process bus. Therefore, the transfer delay time varies depending on congestion statuses. Therefore, even if an average transfer-delay time of the time-limit operation command is corrected, a varying part of transfer delay time which depends on congestion statuses of communications appears as a drop of control accuracy.
As has been described above, the conventional breaker-phase-control switching system which employs a process bus causes a transfer delay time in communications. Since the delay time is not always constant, there is a problem that the control accuracy of close or open operation of the circuit-breaker deteriorates.
Accordingly, it is desired to provide a breaker-phase-control switching system with high control accuracy, which employs a process bus, and a circuit-breaker control unit.